Important: This tutorial is outdated and has not been updated since Mount & Blade v.0.731. While some of the information provided may be useful, it is not intended as an active tutorial. Use at your own risk.
Notice: This tutorial is for creating new items in an existing mod, by editing existing textures. You'll need the following programs to start:
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BRFview (by Lurb)
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Unofficial Editor (by Effidian)
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Texture & Material Editor (by Simpson)
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Adobe Photoshop (by Adobe)
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Adobe Photoshop DDS Plug-in (by Nvidia)
Mount & Blade Tutorial: Creating New Items[/u]
Editing a texture: (C:/Program Files/Mount&Blade/Textures/..)
1.) Load a selected DDS texture into Photoshop, choosing not to use existing MIP maps. (i.e. costumes4.dds)
2.) After editing the texture, save it as a unique DDS file and generate MIP maps. (i.e. costumes4_new.dds)
Creating a BRF mesh file:1.) Launch BRFview.
2.) Load file item_meshes1.brf.
3.) Find the mesh for the item you've created a new texture for.
4.) Delete every other mesh in that BRF file, except the one tied to your texture.
5.) Save the BRF file.
Creating texture and material BRF files:1.) Launch Simpson's Texture and Material Editor.
2.) Create a new texture BRF file, listing your new texture file name.

3.) Save it as a unique BRF file. (i.e. textures_new.brf)
4.) Create a new material BRF file, referencing the new texture.

5.) Save it as a unique BRF file. (i.e. materials_new.brf)
Editing your existing BRF mesh file:1.) Go into your Mount & Blade CommonRes folder. (../Mount&Blade/CommonRes)
2.) Back up these files: textures.brf and materials.brf
3.) Back up your existing BRF files: textures_new.brf and materials_new.brf
4.) Re-name your existing BRF files to override those in the CommonRes folder. (i.e. change the names to match)
5.) Replace the textures and materials BRF files with the new ones.
6.) Launch BRFview.
7.) Load the edited version of item_meshes1.brf. (i.e. whatever you named your mesh file)
8.) Set the material of your mesh to the name used in materials_new.brf.
9.) Save the file.
CommonRes return:1.) Move the backed up version of textures.brf and materials.brf back to the CommonRes folder, replacing your edited version.
Loading the new items: (../Mount&Blade/Module/..)
1.) Open module_info.txt.
2.) Edit the TXT file to load your new BRF files.
Folder placement:1.) Make sure all your BRF files are in the following folder: ../Mount&Blade/Module/../Resource
2.) Make sure all your DDS files are in the following folder: ../Mount&Blade/Module/../Textures
After that, you should be all set to run Mount & Blade.
Special thanks to Janus, who basicly taught me the general step-by-step process of creating new items.Well, the process you went through provides the new meshes and textures, but doesn't add an item that actually uses them. You'll need to add (or copy) an item in the Unofficial Editor, switch to the "Meshes & Modifiers" tab, and change the mesh for it to your newly added one.